Hydrogenation of hydrocarbons with catalytic microspheres

ABSTRACT

Hydrocarbon fractions boiling below 975* F. are obtained by hydrocracking heavy hydrocarbon oils, containing 40-100 percent by volume of hydrocarbon fractions boiling above 975* F., in the liquid state in the presence of ebullated catalytic macroporous microspheres. These microspheres are smaller than 60 mesh (U.S. Standard) and larger than 325 mesh. They have a pore volume of at least 0.10 cc./g. in pores larger than 250 Angstroms (A) and at least 0.30 cc./g. in pores less than 250 A. The macroporous, microspheres have an average size such that 80 weight percent fall within a narrow size range and are ebullated by the upward flow of oil and hydrogen through the reactor during hydroconversion. The pore volume of the microspheres is critical as there must be a penetration of the hydrocarbon oil into the catalyst for at least a 3 percent gain in weight.

Unite States Patet [7 2] Seymour 8. Alpert Princeton; Michael C.Chervenak, Pennington; Ronald B. Wolk, Lawrence Township, Mercerlnventors [54] HYDROGENATION OF HYDROCARBONS WITH CATALYTIC MICROSlHERES5 Claims, 1 Drawing Fig.

[52] U.S.C1 208/111, 208/110,208/1l2, 208/216, 208/217 [51] Int. Cl..10g 13/02, CiOg 13/ 1 8 [50] Field of Search 208/112, 1 10, ll 1 [56]References Cited UNITED STATES PATENTS 3.183.180 5/1965 Schuman et a1.208/110 3,235,508 2/1966 M1115 .l 208/112 3,412,010 11/1968 Alpertetal208/112 3,477,944 11/1969 VanDriesen 208/112 Primary Examiner-Curtis R.Davis Alt0rneysNathaniel Ely and Bruce E. Hosmer AfiSTRACT: Hydrocarbonfractions boiling below 975 F. are obtained by hydrocracking heavyhydrocarbon oils, containing 40100 percent by volume of hydrocarbonfractions boiling above 975 F., in the liquid state in the presence ofebullated catalytic macroporous microspheres. These microspheres aresmaller than 60 mesh (US. Standard) and larger than 325 mesh. They havea pore volume of at least 0.10 cc./g. in pores larger than 250 Angstroms(A) and at least 0.30 cc./g. in pores less than 250 A. The macroporous,microspheres have an average size such that 80 weight percent fallwithin a narrow size range and are ebullated by the upward flow of oiland hydrogen through the reactor during hydroconversionl The pore volumeof the microspheres is critical as there must be a penetration of thehydrocarbon oil into the catalyst for at least a 3 percent gain inweight.

PATENTEDNUV 23 I971 v 3,622,500

Conversion with Microspheres(Diome1ers of 2000 A) 0 I0 20 3O 4O 50 6O 708O 9O Conversion with Exnudcnes INVENTORS SEYMOUR B.ALPERT MICHAELC.CHERVENAK RONALD H. WOLK by): a d? AITTOR Y HYDROGENATION OFHYDROCARBONS WITH CATALYTIC MICROSPI-IERES BACKGR OUND OF THE INVENTIONThis invention relates to the hydroconversion of heavy hydrocarbon oils,and more particularly to such treatment of oils so as to effecthydrocracking and hydrodesulfurization in the presence of ebullatedcatalyst particles.

l-lydroconversion is the conversion of heavy hydrocarbon oils containingsignificant amounts of the extremely high molecular weight compounds tothe more valuable hydrocarbon fractions which boil below 975 F.Hydroconversion of hydrocarbon oils is disclosed and claimed with theaid of ebullated catalyst particles in US. Pat. No. Re. 25,770. Whilethis patent used catalyst particles larger than 60 mesh size (TylerMesh), US. Pat. No. 3,183,180 teaches the hydroconversion of hydrocarbonoils in the presence of ebullated catalysts having particle sizes in therange extending to 325 mesh (Tyler Mesh).

The advantage of using a catalyst having a small particle size is thatas the catalyst particles decrease in size, the available externalcatalyst surface area increases. This increase in external catalystsurface area improves the conversion of reactants in the reaction zone.At the same time that the catalyst size is decreased, consideration mustbe given to the catalyst pore volume and pore size in view of themolecules involved in the reaction. Increasing the catalyst surface areaby decreasing the catalyst particle size in order to obtain improvedconversions will not be enough where the molecules in the reaction areof high molecular weight and the pore size is small. The high molecularweight molecules block the small pores of the catalyst and this resultsin a decrease in conversion due to the lost catalyst surface area.

There are certain heavy hydrocarbon oil feedstocks which can becharacterized by the presence of a small amount, on the order of 1percent or more, of very high molecular weight compounds. Vacuum residuacontaining at least 1 percent of such high molecular weight compoundswould be obtained from the Kuwait, Khafji, certain Mid-Continent, andVenezuelan oil feeds. These high molecular weight compounds are verylarge in comparison to most of the pores in the catalyst and underreaction conditions the catalyst pores become blocked. If, for example,one should operate with an extruded catalyst as taught by Re. 25,770,the catalyst will become surface pore blocked after some limited useageeven when the catalyst contains many macropores. This is a result of theopenings to the macropores existing only at the surface of theextrudate. Reducing the size of a given extrudate will not provesufficient for successful operation of the residuum conversion processunless the finely divided extrudate catalyst also has many macropores.It is the right combination of a finely divided, closely sized catalystparticle having an adequate number of macropores which is necessary forthe optimum ebullated bed hydroconversion operation of residuum feedscontaining some amount of large molecular weight compounds. Macroporousmicrospheres allow operation at conditions that are more severe thanthose at which microporous and macroporous extrudates can be used. Theadvantage to be found in such macroporous microsphere operation is amarked increase in conversion of the feed.

SUMMARY OF THE INVENTION In accordance with this invention, an improvedprocess for the hydroconversion of heavy hydrocarbon oils is carried outin the presence of an ebullated bed of catalyst particles in the form ofmacroporous microspheres of a size smaller than 60 mesh and larger than325 mesh (U. S. Standard).

The catalytic macroporous microspheres are composed of the metals,oxides, sulfides or salts generally recognized to have activity forhydrogenating oils. Suitable metals such as platinum or palladium, andthe oxides and sulfides of molybdenum, nickel and cobalt are frequentlyused in the hydrocracking of oils. The catalytic components are commonlysupported on carriers such as alumina, silica and mixtures thereof.Additionally, the catalytic composition may include a small amount of anadjuvant such as a fluoride or a chloride.

To facilitate the maintenance of a well ebullated bed of catalyticmacroporous microspheres, it is highly desirable to use macroporousmicrospheres that differ only slightly in average size. It has beenfound that best results are obtained when at least about percent byweight of the catalytic macroporous microspheres fall within a narrowsize range, such as 60 to 100 mesh, (U. S. Standard), 100 to 200 mesh or140 to 325 mesh.

Both for good ebullation of the catalytic macroporous microspheres andfor satisfactory residence time of the heavy oil in the reaction zone,the feedstock is passed upwardly through the reaction zone at a spacevelocity (volume of oil per hour per volume of reaction zone) in therange of about 0.2 to 3.0 and preferably in the range of about 0.4 toabout 1.5. Simultaneously, hydrogen is passed upwardly through thereaction zone at a higher velocity than that of the feedstock, the flowsof liquid and gas being controlled so that the oil velocity does notexceed 0.4 of the gas velocity. Preferably, the liquid velocity will bein the range of about 0.04 to about 0.15 of the gas velocity.

Fresh feed hydrogen is at least about percent by volume pure andpreferably is over about percent by volume pure. The diluents of thefresh feed hydrogen are usually carbon oxides, nitrogen, argon, methaneand H 0. To achieve desirable rates of hydrogenation of heavyhydrocarbon oils pursuant to this invention, the pressure in thereaction zone is maintained in the range of about 1,000 to about 5,000p.s.i.g. (pounds per square inch gauge). Generally, it is preferred tocarry out the hydroconversion of heavy oils at a pressure in the rangeof about 1,500 to about 3,000 p.s.i.g.

The hydroconversion of heavy oils is generally conducted at atemperature in the range of about 750 to about 900 F. In most cases, atemperature in the range of about 800 to about 850 F. is preferred.

The advantages of this invention are that heavy oils with over 40percent by volume and as much as percent by volume thereof boiling abovea temperature of 975 F. can be converted to at least 50 percent byvolume boiling below 975 F. to more valuable lower boiling hydrocarbons,while maintaining the activity of the catalytic macroporous microspheresover a longer period of operation than has previously been foundpossible heretofore with microporous microspheres or either extrudedmicroor macroporous catalysts. Even with feedstocks that contain 100percent by volume hydrocarbons boiling above 975 F., conversions ofabout 70 to about 90 percent by volume to lower boiling hydrocarbons arefrequently attained.

Accordingly, the principal object of this invention is to provide animproved process for the hydroconversion of hydrocarbon oils in thepresence of an ebullated bed of microspherical catalyst particles whichdo not have the limitations and disadvantages of extruded catalysts.

Another object of this invention is to provide an improved process forthe hydroconversion of heavy hydrocarbon oils containing at least 1percent of very high molecular weight compounds in the fraction boilingabove 975 F., wherein the hydrocarbons can be hydrocracked to convert atleast 50 percent by volume of the fraction boiling above 975 F. to morevaluable lower boiling hydrocarbons.

Yet another object of this invention is to provide an improved processfor the hydroconversion of heavy hydrocarbon oils in the presence ofcatalyst microspheres.

A further object of this invention is to provide an improved process forthe hydroconversion of heavy hydrocarbon oils using an ebullated bed ofmacroporous catalyst microspheres.

Further objects of this invention will become apparent from the detaileddescription of the preferred embodiment of this invention followingherein.

The FIGURE is a graph showing the effects of using macroporousmicrospheres versus extrudates in hydroconversions.

DESCRIPTION OF PREFERRED EMBODIMENTS The advantages of the process ofthis invention are particularly unexpected when using feedstocks of thetype that ordinarily deactivate the catalyst at a relatively high rate.Some exploratory studies using gel permeation chromatography indicatethat such feedstocks contain at least 1 percent by weight ofhydrocarbons of large molecular size, exceeding 1,000 Angstroms. Afeedstock with an API gravity of 10.50 and with only about 1 percent byweight of hydrocarbons with molecular sizes ranging from 10,000 to20,000 Angstroms cannot practically be converted in the presence of anebullated bed of microporous microspheres or extruded catalyst to effecteven a minimum conversion of 50 percent by volume of the hydrocarbonsboiling over 975 F. to lower boiling hydrocarbons without fouling thecatalyst particles at such a rapid rate that in a short period, on theorder of a few days, the catalyst is essentially deactivated. Yet thesame feedstock when hydroconverted in the presence of an ebullated bedof catalytic macroporous microspheres is converted to the extent that atleast 50 percent by volume of hydrocarbons originally boiling above 975F. is recovered as valuable lower boiling hydrocarbons while thecatalytic macroporous microspheres maintain an economically acceptableactive life.

The advantages of the process are particularly applicable to feedstockscontaining small amounts of high molecular weight compounds.Conventional inspections and gel permeation chromatographs wereperformed in two feeds (A and B) with the following results:

TABLE I Feed A B Gravity, 'API |0.5 5.8 Sulfur wt. L36 4.02 Viscosity,Saybolt Furrol at 210 FJsec. 808 2,039 Ramsbottom Carbon, wt. 1: 18.821.4 Pentane Insolubles, wt. [7.1 20.7 Percent Boiling Below 975 F. 1.73.9 Average Molecular Weight 1,000 L033 GEL PERMEATION CHROMATOGRAPHICANALYSES As may be seen, the gel permeation chromatograph reveals 10-20percent of the A feed as high molecular weight compounds which wouldtend to block small size pores in regularly used catalysts.

The drawing compares the conversion of the fractions boiling above 975F. to fractions boiling below 975 F. when using amacroporous microspherecatalyst and when using an extrudz te catalyst. The curve shows thatwhen operating the microsphere or the extrudate systems at the sameoperating conditions, a conversion, such as 33 percent using an extrudedcatalyst would result in an improved conversion under those sameconditions to 50 percent by using a macroporous microspheres catalyst.

In this invention it has been discovered that the macroporousmicrosphere catalyst must have certain critical characteristics. One ofthese is a penetration number as hereinafter set forth. As the abilityof the feed to penetrate a catalyst is. dependent on the porosity of thecatalyst it was additionally discovered that the pore structure of themicrospheres was important.

A macroporous microsphere catalyst according to this invention should beof a type and fall within a given size range as hereinbefore described.At the same time the macroporous microspheres have a pore volume of fromabout 0.10 to 0.60 cc./g. comprising pores larger than 250 A and a porevolume of from about 0.30 to 0.50 cc./g. comprising pores with adiameter of less than 250 A. The total pore volume of the macroporousmicrospheres is between about 0.40 and about 1.10 cc./g. A preferredmacroporous microsphere catalyst would have a pore volume of from about0.2 to about 0.4 cc./g. in pores with a diameter larger than 250 A and apore volume from about 0.35 to about 0.45 cc./g. in pores with adiameter of less than 250 A and the total pore volume is between about0.55 and about 0.85 cc./g.

In this invention it has been discovered that previously known porositytests are misleading and indeterminative for commercial catalysts.So-called BET numbers must be correlated with the actual conditionsunder which they are determined, and these, basically, comprise, singly,or in combination: (1) mercury penetration, (2) nitrogen penetration and(3) commercial solvent (e.g. benzene, toluene, isopropyl alcohol)penetration. None of these is indicative of field acceptability of acatalyst.

A new test which can be used with most commercially available porositymeters to determine catalytic life and efficiency was developed. Thisdevelopment was based on the realization that tests of catalysts usingultra-high surface-tension agents, ultra-low surface-tension agents,commercial solvents or the like are inadequate. This stringent testaccurately rates potential catalytic efficiency and life expectancy bytesting porosity with a combined deleterious agent and common diluentmaterial.

Basically the penetration test consists of mixing about 1 to 2 grams ofthe catalyst microspheres with about 5 grams of an equal proportionmixture of oil and benzene in a 50 milliliter beaker for 15 minutes. Theslurry is filtered in a tared Gooch crucible using benzene until thefiltrate is clean appearing. The crucible and microspheres are dried ina vacuum oven at l00 C. for 1 hour. After the microspheres and cruciblehave cooled, they are weighed to determine the weight gain which is adirect indication of oil penetration.

Microscopic inspection or the like is used to physically measure thepercent penetration which has occurred on catalyst sample. A distinctcolor differential exists between that portion of the catalyst which hasbeen penetrated and that portion in which no penetration has occurred.

There is a good correlation between the penetration numbers and theefficiency, overall operability, and catalyst life in residuumhydroconversion and the like processes. This test has the distinctadvantage over pervious porosity measurements in that it reflects thoseparameters of the catalyst which are most important under actualoperating conditions. The previously known tests do not reflect catalystparameters such as constrictions, orientation, passage size and thelike.

It is desirable that the penetration number he at least 3 percent and itis preferable that it be greater than 5 percent but it is most preferredthat it be greater than 10 percent for a macroporous microsphericalcatalyst with 100 percent penetration being ideal.

It has now been found that where an extruded catalyst in an ebullatedstate with a given heavy oil is limited to about percent volumeconversion of hydrocarbons originally boiling above 975 F. to lowerboiling hydrocarbons, the conversion can be increased at least 5(absolute) to 95 percent simply by using catalytic macroporousmicrospheres in place of the extruded catalyst. This type of feedmaterial is usually free of the very high molecular size compounds.However, in cases where extruded catalysts are only capable of effectingin various heavy oils between about 30 and about 80 percent conversionof hydrocarbons originally boiling above 975 F. to lower boilinghydrocarbons, the replacement of the extruded catalyst by catalyticmacroporous microspheres in each case results in a substantial increaseof about to about 20 percent (absolute) in the conversion ofhydrocarbons originally boiling above 975 F. to lower boilinghydrocarbons. in general, the lower the conversion of a heavy oil is inusing an extruded catalyst, the greater the increase in conversion iswhen catalytic macroporous microspheres are substituted for the extruded catalyst.

As taught in the aforesaid patents, an ebullated bed is one in which thesolid particles are kept in random motion by the upward flow of liquidand gas; the ebullated bed will have a gross volume at least 10 percentgreater than the volume of the solids thereof in a settled state. Thebulk density of fresh catalytic macroporous microspheres used in thisinvention is in the range of about 25 to about 60 pounds per cubic foot.Even with macroporous microspheres of lowest bulk density, theconcentration of the macroporous microspheres in a well ebullated bed isgreater than 5 pounds per cubic foot while above the interface whichdefines the top of the ebullated bed, the concentration of themacroporous microspheres is less than 0.] pound per cubic foot.

As previously mentioned, catalytic macroporous microspheres exhibit acomparatively long active life. Generally, replacement of the catalyticmacroporous microspheres in an ebullated hydrocracking zone takes placeat a rate in the range of about-0.05 to about 0.25 pound per barrel (42US. gallons) of feedstock. The desired low rates of withdrawal and ofreplacement of catalytic macroporous microspheres are simply effected.At such low rates, fouled macroporous microspheres are withdrawn fromthe hydroconvertor together with converted liquid hydrocarbons whilefresh macroporous microspheres are introduced at the same rate bysuspension in the feedstock being fed into the hydroconversion reactionzone.

The patents also teach the desirability of recycling hydrogen as well asliquid hydrocarbons through the reactor which is of simple construction,usually a cylindrical vessel with a height that is several times thediameter. While the recycling of hydrogen is advantageous in theoperation of the process of this invention, the recycling of liquidhydrocarbons is only occasionally necessary and in such case, the rateof recycled liquid hydrocarbons rarely exceeds one volume of recycledliquid per volume of feedstock supplied to the hydrocracker.

Having thus described the invention in general terms, reference is nowmade to the specific examples which have been carried out in accordancewith the techniques of the present invention which should not beconstrued as unduly limiting thereof.

EXAMPLE 1 A feedstock composed of equal volumes of the residuum of avacuum distilled crude oil and of the distillation residuum of adeasphalted oil consisted entirely of hydrocarbons boiling above 975 F.Catalytic macroporous microspheres entirely in the size range of 70 to325 mesh with 84 percent by weight thereof being in the size range of100 to 270 mesh were compared with an extruded catalyst of the samecomposition, the extruded catalyst being in the form of tiny cylindershaving a diameter of one thirty-second inch. Using each form of thecatalyst which contained 12.8 percent by weight of molybdenum oxide and3.2 percent by weight of cobalt oxide, the feedstock was hydrocracked inthe presence of an ebullated bed of that catalyst under comparableconditions and with the results shown in table II.

Herein follows the porosity of the catalysts used:

Macroporous Porosity Microspheres Extrudates Less than 250 A 0.35 ccJg.0.4 cc./g. Greater than 250 A 0.30 ccJg. 0.15 cc./g.

TABLE ll Catalyst Extruded Operating Conditions Microspheres CatalystHydrogen Pressure, p.s.i.g. 2,300 2,200 Reaction Temperature, F. 835 837Oil Space Velocity 0.46 1.00 Hydrogen Rate, SCF/Bbl 6,300 6,000

Yields Based on Feedstock H 5, NH,, C C by weight 8.3 6.3 e ase F. byvolume 16.5 9.5 360-650 F. X: by volume 28.8 19.3 650-975 F. by volume35.9 31.8 over 975 F. by volume 24.2 43.8 Total C, and over, by volume105.4 104.4 Successful conversion, by 75.8 volume Failed at indicatedconversion 56.2

'SCF/Bbl is standard cubic feet (measured at atmospheric pressure and 60F.) per barrel of feedstock.

The catalytic macroporous microspheres not only gave a conversion thatwas 19.6 percent (absolute) higher than that obtained with the extrudedcatalyst, but also produced a higher proportion of highly desirableliquid hydrocarbons boiling below 650 F.

EXAMPLE 2 A Canadian residuum of 10 API gravity boiling entirely above975 F. and containing 4 percent by weight of sulfur was hydrocrackedwith each of the catalysts of example 1 in an ebullated state. Table IIIpresents the comparable operating conditions and the results obtained.

Fuilcd at indicated conversion ",4 35.0

EXAMPLE 3 The residuum of a Middle East crude oil known to have a highpropensity to deposit carbon on hydrogenation catalysts was hydrocrackedwith the catalysts of example 1 in an ebullated state. Table IV givesthe operating conditions and the results obtained with this feedstockhaving only 40 percent by volume thereof boiling above 975 F., and APIgravity of 9 and a sulfur content of 5 percent by weight.

EXAMPLE 4 Comparative tests were run on Kuwait Vacuum Residuum (7AP],5.5 W %S) on two different types of microspheres, only one of which hadthe proper macroporosity as set forth hereinbefore. The operatingconditions and the yields are summarized below. It should be noted thatthe maximum conversion on the macroporous catalyst was at least 86.5percent. The nonmacroporous catalyst failed at 77.6 percent.

Mucroporous Mncroporous Microspheres Microspheres Pore Volume In re! 250A cc./g. 0.13 0.07 Operating Conditions Hydrogen Pressure, p.s.i.g.2,000 2,000 Reaction Temperature, F. 833 832 Oil Space Velocity 0.490.75 Hydrogen Rate, SCF/Bbl 6,000 4.000 C.C H,S NH, Wt. 8.5 9.1 C,-400F. by volume 28.5 21.3 400-680 F. by volume 35.9 32.6 680-975 F. k byvolume 29.6 30.5 over 975 F. by volume 13.5 22.4 Total C. and over Z: byvolume 107.5 106.8 95 Successful conversion. 17 by volume 86.5

Failed at indicated conversion 77.6

Those skilled in the art will visualize many modifications andvariations of this invention without departing from its spirit andscope. Accordingly, the claims should not be interpreted in anyrestrictive sense other than that imposed by the limitations recitedwithin the claims.

We claim:

1. In a process for the hydroconversion of a hydrocarbon feed containingat least 40 percent by volume of components boiling above 975 F. whereinthe feed and a hydrogen-rich gas pass upwardly through a reaction zonecontaining a bed of particulate catalyst selected from the groupconsisting of platinum, palladium, molybdenum, nickel, cobalt, oxidesand sulfides thereof and mixtures thereof supported on a carrierselected from the group consisting of alumina, silica and mixturesthereof; while said reaction zone is maintained at a temperature between750 and 900 F. and at a pressure between l,000 and 5,000 p.s.i.g. with acombined fluid velocity sufiicient to expand the bed of catalyst so asto maintain the catalyst in random motion in the liquid without substantal carryover of the catalyst from said reaction zone the improvementwhich comprises:

a. using a particulate macroporous catalyst having a total pore volumebetween 0.4 and 1.1 cc./g.;

1. said catalyst having a pore volume between 0.1 and 0.6 cc./g. inpores with diameters greater than 250 A and; 2. having the remainder ofthe pore volume comprised of between 0.3 and 0.5 cc./g. in pores withdiameters less than 250 A and; 3. the oil penetration number being atleast 3 percent;

b. and maintaining a space velocity such that at least 50 percent ofsaid components boiling above 975 F. are converted to components boilingbelow 975 F.

2. The process of claim 1 wherein the particulate catalyst issubstantially spherical having a size such that percent of said catalystfalls within a narrow size range between 60 and 325 Tyler mesh.

3. The process of claim 1 wherein the oil penetration number is at least30 percent.

4. The process of claim 1 wherein the improvement comprises using amacroporous microsphere catalyst:

a. said catalyst having a pore volume between about 0.2 and 0.4 cc./g.in pores with diameters greater than 250 A and;

b. having a pore volume of between about 0.35 and about 0.45 cc./g. inpores with diameters less than 250 A and;

0. having a total catalyst pore volume between about 0.55

and about 0.85 cc./g. and;

d. having an oil penetration number of at least 10 percent.

5. The process of claim 4 wherein said catalyst is cobaltmolybdate onalumina.

i I l

2. The process of claim 1 wherein the particulate catalyst issubstantially spherical having a size such that 80 percent of saidcatalyst falls within a narrow size range between 60 and 325 Tyler mesh.2. having the remainder of the pore volume comprised of between 0.3 and0.5 cc./g. in pores with diameters less than 250 A and;
 3. the oilpenetration number being at least 3 percent; b. and maintaining a spacevelocity such that at least 50 percent of said components boiling above975* F. are converted to components boiling below 975* F.
 3. The processof claim 1 wherein the oil penetration number is at least 30 percent. 4.The process of claim 1 wherein the improvement comprises using amacroporous microsphere catalyst: a. said catalyst having a pore volumebetween about 0.2 and 0.4 cc./g. in pores with diameters greater than250 A and; b. having a pore volume of between about 0.35 and about 0.45cc./g. in pores with diameters less than 250 A and; c. having a totalcatalyst pore volume between about 0.55 and about 0.85 cc./g. and; d.having an oil penetration number of at least 10 percent.
 5. The processof claim 4 wherein said catalyst is cobalt-molybdate on alumina.